Over the last two decades umbilical cord blood (UCB) transplantation (UCBT) is increasingly used for a variety of malignant and benign hematological and other diseases. The main factor that limits the use of UCB to low weight recipients, mainly children and adolescents, is its low progenitor cell content. Various alternatives have been exploited to overcome this difficulty, including the transplantation of two UCB units (double umbilical cord blood transplantation, dUCBT). Following dUCBT, donor(s) hematopoietic stem cells (HSC) can be detected in the peripheral blood of the recipient as soon as 14 days post-transplantation. Sustained engraftment of HSC from one or both donors can be observed- dominance or mixed chimerism respectively, although single donor unit dominance has been observed in over 85% of patients. The underlying biology, which accounts for the interactions both between the two infused UCB units- cooperative or competitive, and with the recipient's immune system, has not been elucidated.